Titanium alloys are light in weight and have high strength and are therefore used in various fields of parts in which low weight is important, such as aircraft parts and automobile parts. Titanium alloys are also superior in corrosion resistance and biocompatibility and are also widely used in the field of biological implant devices. In any of these fields, α-β type titanium alloys, typically exemplified by Ti-6Al-4V, are common because the alloys have high strength and broad utility.
In view of these circumstances, development of increased strength in α-β type titanium alloys that have high practical utility due to low cost are actively pursued. For example, patent publication No. 1 below discloses a technique in which Ti-6Al-4V is subjected to gas nitriding, and a brittle TiN compound surface layer is removed, thereby improving fatigue strength. Patent publication No. 2 below discloses a technique in which a first layer of a nitrogen solid solution hard layer and a second layer of an oxygen solid solution hard layer are formed simultaneously on pure titanium or Ti-6Al-4V, thereby hardening a surface of the member. Patent publication No. 3 below discloses a composite material in which a TiC compound is dispersed in Ti-6Al-4V.
On the other hand, β type titanium alloy can be mentioned as the high strength titanium alloy. However, the β type titanium alloy contains a great amount of rare metals and is more expensive than the α-β type titanium alloys. Furthermore, in the β type titanium alloy, the static strength can be improved by age (precipitation) hardening, but the fatigue strength is not proportional to the static strength and is not sufficiently improved. This is because precipitated phase with high hardness is generated by the age hardening treatment and improves the static strength, but has a great difference in the hardness (or elastic strain) from the matrix primarily made of the β phase. Thus, for fatigue caused by repeated stresses, a boundary between the precipitated phase and the β phase tends to be origins of fractures.
The above-mentioned patent publications are as follow.    1: Japanese Unexamined Patent Application Laid-Open No. Hei5 (1993)-272526    2: Japanese Unexamined Patent Application Laid-Open No. 2000-96208    3: Japanese Patent No. 4303821
According to the techniques disclosed in the patent publications Nos. 1 and 2, only the surface of the member is strengthened, and the inside of the member is difficult to strengthen. That is, the techniques are effective for improving wear resistance and preventing fatigue crack formation on the surface, but are less effective for improving static strength and preventing fatigue crack growth. In the technique disclosed in the patent publication No. 3, a titanium alloy powder and a TiC compound powder are mixed together, compacted, and then sintered. It is difficult to uniformly mix powders which have different specific gravity, and the metallic structure after the sintering is therefore not uniform. That is, low-strength portions may exist and decrease reliability of strength as a member and quality stability, and thereby the sintered compact is difficult to produce as industrial products practically.
In the technique disclosed in the patent publication No. 2, the alloy contains a second layer of an oxygen solid solution hard layer in which oxygen is an α-stabilizing element as well as nitrogen. Although oxygen is an α-stabilizing element as well as nitrogen, oxygen easily forms a hard and brittle α case (α-stabilizing element rich layer) compared to nitrogen. Therefore, it is difficult to stably control the formation of the oxygen solid solution hard layer in a production process. It is generally known that the action of oxygen for high strengthening is less than that of nitrogen.